def say_hello(user = 'world'):
  ''' 一个简单的函数 '''
  print('hello',user.title())

def get_double(num):
  return num * 2

user = 'xixihaha'

class Dog():
  def __init__(self, name, age):
    self.name = name
    self.age = age

  def sit(self):
    '''模拟蹲下'''
    print(self.name.title(),'is now sitting')
  def roll_over(self):
    '''打滚'''
    print(self.name.title(),'rolled over')
